This invention relates to solar energy-assisted drying, eg. dehydration, typically tobacco drying, titanium dioxide drying, onion dehydrating, polyester fiber heat setting, nut roasting, and breakfast cereal toasting. Thus, the term "drying" herein is used broadly whether or not much, if any, water or other liquid is removed from the goods in process; in some cases such operation is accompanied by a toasting or cooking, and in many other cases such change in product character is guarded against by careful control of temperature and drying rate.
Heretofore many goods including foods were dried by exposure to the sun's rays in the presence of adequate ventilation. Typical continuous through circulation dryers now conventionally used for drying vegetables, fruits, etc., have a natural gas burner which discharges its hot combustion into a mixture of recycled vapors from the drying bed and makeup air from the outside; the resulting hot gas and vapor mixture is drawn with a fan from a gas mixing plenum and blown into drying contact, eg. downwardly or upwardly through the gas-permeable bed of the goods to be dried. Such bed usually is supported by a conveyor or truck line passing transversely to the flow of this warm gas. The gas is withdrawn from the drying contact, and most of it is recycled for reheating (and, thus, dehumidification) while a portion is exhausted to atmosphere as spent gas. While there are many other types of continuous through circulation dryers, these are well known to those skilled in the art and will not be dealt with here. It is well known also in the operation of such dryers to recoup heat by exchanging the heat of the spent gases with the incoming makeup air to the dryer (frequently using heat wheels, indirect heat exchangers, regenerative exchange, and the like).
Solar air heaters (collectors) are not new, but have come into recent popularity as a result of the desire to conserve petroleum and natural gas. Solar heater (collector) units especially useful in connection with the present invention are shown in Australian Pat. No. 276,788, published on Nov. 26, 1965, and U.S. Pat. No. 3,908,631 of Sept. 30, 1975. If the collectors are rigid, they need not have auxiliary blowers to inflate them for the instant purpose, although these can be used if desired.
Advantages of the present invention over prior proposals include: the fact that solar energy is used only to augment the drying process, and then in a way for obtaining practical solar heating at very low capital cost, a prime consideration even in high fuel cost areas; and the solar energy need not be stored in the system at the expense of building storage facilities, but can be used directly for effecting fuel savings during sunlit hours; the solar collector is disposed for obtaining high air flow rates and only modest temperature rise. This makes the instant apparatus particularly economic in comparison to other proposed solar heating systems, particularly where the air or gas flow therethrough is minimized and its temperature maximized, making the invention especially adaptable to use low temperature dehydration processes.